Normally, present-day color cathode ray tubes are constructed such that an electron beam focus electrode requires a relatively low value of focus potential for proper operation of the cathode ray tube i.e. 4-5 KV for example. Also, it is a common practice to couple the above-mentioned focus potential from a source into the cathode ray tube by way of a pin located in the base of the cathode ray tube.
However, some recently designed cathode ray tubes, designated as tri-potential in-line cathode ray tubes, have an additional focus electrode which requires a voltage potential in the vicinity of about 12 KV. Such structures have been found to provide a sharper image reproduction under relatively high electron beam current conditions.
As a result of the above-described increased value of potential applied to the added focus electrode of the cathode ray tube, it becomes necessary to increase the spacing or isolation of the base pin receiving the increased focus potential from the other base pins to prevent undesired and untolerable arcing between the base pins of the cathode ray tube. Otherwise, the increased probability of arcing due to the increased value of potential applied to the newly added focus electrode presents a very possible area of catastrophic component failure of relatively sensitive circuitry coupled to base pins adjacent to the focus electrode base pin.
In considering the problem of undesired arcing between the base pins due to the increased value of potential applied to the base pin connected to the focus electrode, it is noted that a separate base pin has normally been employed for each screen grid electrode, for each control grid electrode, and for each cathode electrode of multiple grid cathode ray tubes. Such a convenience allowed matrixing of the chrominance and luminance signals within the color cathode ray tube and also provided for separate adjustment of potential applied to each screen grid electrode to facilitate proper cut-off bias potential set-up conditions for each electron gun of the color cathode ray tube.
However, one solution to the problem of obtaining added spacing and isolation needed for the base pin carrying the increased value of focus voltage is to eliminate the use of a separate base pin for each one of the screen grid electrodes and the control grid electrodes. By providing only a single grid output and a single screen grid output there is a resultant four unused base pins whose space may be employed to provide the desired increased spacing and isolation for the base pin to which the increased value of focus potential is applied.
Obviously, eliminating separate connections to each one of the screen grid electrodes presents the problem of obtaining a proper set-up condition for both cut-off and color temperature of the color cathode ray tube. However, access to each one of the cathode electrodes suggests a probable approach for effecting both cut-off and color temperature control of the individual electron guns of the cathode ray tube.
In one known form of apparatus for effecting electron gun cut-off and color temperature control of the cathode ray tube via the cathode electrodes, chrominance or color difference signals are AC coupled to an amplifier stage which has a DC restorer circuit coupled thereto for replacement of the DC potential. A switch couples luminance or reference potential signals to the amplifier while both cut-off bias and drive controls are coupled to a common electrode of the amplifier to vary the gain thereof and the potentials applied to the cathodes of the cathode ray tube.
Although the above-described apparatus has provided, or at least permits, an improved utilization of base pins of the cathode ray tube, the complexity of the circuitry and the resultant cost factor appears to be undesirable. Moreover, the common coupling of the adjustable bias controls and color temperature controls provides interaction therebetween which is deleterious to the provision of an improved form of apparatus.